Drilling is the final stage and the supreme arbiter of the exploration process. Knowledge of the subsoil acquired through geological and geophysical surveys allows the potential of a prospect to be broadly evaluated, but cannot definitely confirm the presence of suspected hydrocarbon resources. Certainty can only be obtained by gaining direct access to the subsurface through drilling. Drilling also provides prospectors with a range of valuable data on the lithology and fluids present.
Chapter 2Oil and gas exploration and production
Figure 2.9 Drill bits.
Drilling an exploration well can take several (2 to 6) months, but the precise duration is difficult to predict because of geological uncertainties at this level. Important doubts will always remain about the depths, the hardness of the rocks and interstitial pressures in the formation, which can only be swept away by drilling. On average one drilling in five results in the discovery of an economically feasible hydrocarbon reservoir. This falls to 1 in between 7 and 10 in relatively unexplored zones.
2.2.4.2 Principles of drilling
The objective of drilling is to create a link between the surface and the target formation by penetrating the various geological strata down to a depth of up to ten kilometres (35 000 ft).
The most widespread technique involves attacking the rock with a rotating drilling bit (Fig. 2.9). Three factors are involved in this process: the weight exerted by the drilling bit on the rock, its rotation and the removal of the cuttings using a circulating fluid (the drilling mud).
Chapter 2Oil and gas exploration and production
Figure 2.10 Main components of a rig.
Crown block
Drilling cable
Travelling block
Hook
Injection head
Rotary table Drill pipe
Drilling winch
Mud pumps
The drilling bit is attached to a drillstring made up of tubular elements which are screwed on as the drilling advances: drill-pipes and drill-collars close to the bit. This assembly is suspended and manipulated from a derrick (Fig. 2.10). Depending on the type of well the rotary movement is generated either:
– From the surface by means of a rotary table and a transmission pipe known as a kelly, or by a power swivel connected directly to the last drill-pipe; or
– At the bottom of the well only, by means of a drilling turbine or engine (turbodrilling).
In addition to cleaning the bottom of the well, drilling mud helps to cool and lubricate the drilling bit, to consolidate the walls of the wellbore and exercise pressure such as to contain the flow of oil, gas or water from a drilled formation.
Drilling starts with a large bit, for example of 26 in. (66 cm) in diameter attached to a drill-collar and a drill-pipe. When drilling has reached a certain depth a new drill-pipe is added to the drillstring. This procedure is repeated each time the increase in drilled depth reaches the length of a drill-pipe, until a certain depth is reached, when the wellbore is cased.
Lengths of steel casing of diameter corresponding to that of the wellbore are lowered into the wellbore one at a time, and cemented in place so as to protect the groundwater and control fluids emitted from the well. Several items of equipment are fitted to the upper extremity of the casing to insure suspension and seal the opening. Safety devices known as blow-out preventers are also fitted at the wellhead, fitted with high pressure valves which allow the well to be sealed rapidly using remotely controlled valves in the event of a sudden surge.
The casing and other equipment are subjected to a series of pressure tests, and if the requisite safety requirements are all met the next drilling stage can begin. A new drilling bit of smaller diameter is lowered into the hole inside the surface casing, and operations proceed in the same manner as before. When a certain depth has been reached the hole is again cased using smaller casing which matches the diameter of the new hole. The size of the drilling bit is again reduced, the procedure is repeated, and so on. As drilling progresses, successively smaller drill bits are used and the diameter of the cased hole decreases, as shown in Fig. 2.11.
Drilling proceeds at a rate of several metres per hour, the rate declining with increasing depth, punctuated by difficulties and the need to regularly replace the drilling bit, which involves withdrawing the entire drillstring As drilling advances a drilling log is maintained in which information is entered regarding the drilled depth, the nature of the rock and the fluids encountered, the drilling durations and any noteworthy events. This document is of great value to geologists and geophysicists.
2.2.4.3 Choice of drilling equipment
For onshore exploration the choice of drilling rigs depends on the target depth, access facil-ities to the site and the availability of the derrick. Offshore there is the additional constraint of the depth of water, climatic conditions and the remoteness from the logistical base.
The main difference between onshore and offshore drilling is related to the way in which the rig is supported. Offshore operations are conducted from platforms which either float or are fixed to the sea bed, and which are capable of performing all the functions normally carried out at an onshore drilling site as well as certain other services such as diver support and a meteorology station. The platforms may be either fixed platforms resting on the sea bed, floating structures or semi-submersibles. Self-raising or jackup rigs are generally used in shallow waters. Barges and semi-submersibles with dynamic positioning tend to be kept for deeper waters. These mobile units only remain stationary during drilling, which can last between several weeks and several months (Fig. 2.12).
Chapter 2Oil and gas exploration and production
Chapter 2Oil and gas exploration and production
250
Depth (m)
750
3 600 2 500
3 300 Surface casing
Technical casing 1
Technical casing 2 Concrete
Production casing
Liner hanger
Liner 26" (660 mm)
20" (508 mm)
17 1/2" (444 mm) 13 3/8" (340 mm)
12 1/4" (311 mm) 9 5/8" (244 mm)
8 1/2" (216 mm) 7" (178 mm)
5 3/4" (146 mm) 5" (127 mm)
Figure 2.11 Cased wellbore.
Figure 2.12 Mobile platforms for offshore drilling.
3000 2500 Water depth (m) 2000
1500 1000 500
Jack-up rig
Jack-up rig semi-submersible
Dynamically positioned drill vessel
2.2.4.4 Logging
During drilling, prospectors keep records of a number of physical parameters of the rock and the fluids encountered, known as logs, which they represent graphically as a function of depth or time.
The mud log comprises the various measurements provided by the mud circuit. These include the penetration rate, the characteristics of the drilling mud and the cuttings and cores description. The study of the cuttings brought up to the surface as the drilling progresses, and particularly the cores obtained by replacing the drill bit by a hollow tool known as a core barrel, provides information on the main characteristics of the formations encountered (Fig. 2.13). These relate to the lithology, the fossils present in each stratum (which dates them), porosity, permeability, and fluids saturation.
Chapter 2Oil and gas exploration and production
Figure 2.13 Core samples.
Wireline logging, also commonly known as electrical logging, is carried out during inter-ruptions to the drilling. It uses a tool known as a sonde lowered into the wellbore at the end of an electric cable or wireline. Logging while drilling, on the other hand, is carried out with the help of instruments included in the drillstring (Fig. 2.14).